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[Chicago Astronomer - Astro Joe]

The Three Points needed to be Classified as a Planet

The International Astronomical Union has decided that, to be called a planet, an object must have three traits.

* It must orbit the sun

* Be massive enough that its own gravity pulls it into a nearly round shape -

* Be dominant enough to clear away objects in its neighborhood.

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[Centaur]

Some people are claiming that planets such as Jupiter and Neptune do not clear their neighborhoods. Clearing out the neighborhood was scientific shorthand for what astrophysicist Steven Soter described in a paper he submitted on August 16 to the IAU. The IAU used that paper as the basis of its dynamical qualification for inclusion in the set of planets. Clearing out the neighborhood essentially means possessing more than 100 times the combined masses of other objects that share a planet’s orbital zone.

Here is the abstract from the paper:

A planet is an end product of disk accretion around a primary star or substar. I quantify this definition by the degree to which a body dominates the other masses that share its orbital zone. Both theoretical and observational measures of dynamical dominance reveal a gap of five orders of magnitude separating the eight planets of our solar system from the populations of asteroids and comets. This simple definition dispenses with upper and lower mass limits for a planet. It reflects the tendency of disk evolution in a mature system to produce a small number of relatively large bodies (planets) in non-intersecting or resonant orbits, which prevent collisions between them.

And here is a link to the full paper:

xxx.lanl.gov/abs/astro-ph/0608359

[Chicago Astronomer - Astro Joe]

Let's apply that line of thinking from the thinking to little ole Pluto...

Has not Pluto, in it's own right, "cleared" out it's own orbital zone?
It's not moving about in a debris field - like Ceres is for example, (I still think it qualifies as a planet tho), it has satellites and even a slight atmosphere, even if it's sublimated. Mercury on the other hand possesses none of these attributes, yet it still remains a solid planet. What gives?...

It's not a credible and solid definition the IAU has agreed upon....it's more like Chicago style politics.

[Centaur]

admin said:

It's not a credible and solid definition the IAU has agreed upon....it's more like Chicago style politics.

Have you read Steven Soter's entire 22-page paper? If so, then let me know which points you would like to discuss or contradict. His thesis is quite nicely developed, well written and worthy of one's time. It's the dynamical side of this issue that is not grasped by most of the public. While Soter's intended audience is professional astronomers, he is able to explain the clear dominance of eight planetary bodies for those with at least a minimal understanding of the scientific principles.

Pluto is a KBO. Ceres is an asteroid. Charon is a satellite of a larger body. It makes more scientific sense to classify them together with other objects with similar dynamical histories. This aids in our thinking as we propose theories of the formation, development and future of the solar system.

This whole issue has been affected by the natural conservatism of most human beings. Those who grew up being taught certain things while in grade school are rarely willing to abandon them. Just imagine what it was like during the 16th and 17th centuries when astronomers were proposing that Earth be added to the list of planets. Each piece of scientific evidence was scoffed at as being contrary to common sense. The current Pluto issue shrivels to virtually nothing in comparison with that one. We're still seeing great resistance to the theory Darwin proposed a century and a half ago. Science keeps progressing, despite social obstacles. These mentioned controversies will eventually subside, while new ones will rise to the surface. It's unending, but adds to both the delight and confusion of being a member of our species.

[Chicago Astronomer - Astro Joe]

Definition is NOT consistant for all Planets

One of the three criteria for planethood states that a planet must have "cleared the neighbourhood around its orbit". The largest objects in the Solar System will either aggregate material in their path or fling it out of the way with a gravitational swipe.

Pluto was disqualified because its highly elliptical orbit overlaps with that of Neptune.

But Dr Stern pointed out that Earth, Mars, Jupiter and Neptune have also not fully cleared their orbital zones. Earth orbits with 10,000 near-Earth asteroids. Jupiter, meanwhile, is accompanied by 100,000 Trojan asteroids on its orbital path.

These rocks are all essentially chunks of rubble left over from the formation of the Solar System more than four billion years ago.

"If Neptune had cleared its zone, Pluto wouldn't be there," he added.

Stern said like-minded astronomers had begun a petition to get Pluto reinstated. Car bumper stickers compelling motorists to "Honk if Pluto is still a planet" have gone on sale over the internet and e-mails circulating about the decision have been describing the IAU as the "Irrelevant Astronomical Union".

Source: news.bbc.co.uk/2/hi/science/nature/5283956.stm

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Seems like I am not alone in my belief that the new definition is hokey and just wrong.